This invention relates to radiation detection apparatus and will be described particularly with reference to pyroelectric passive infra-red intruder detection apparatus, that is, to apparatus responsive to infra-red radiation emitted by an unauthorised entrant into a space at a time when the space should be empty.
Such apparatus works on the principle that a change in infra-red radiation within its field of view is detected by the apparatus. The change in detected radiation produces an electrical signal which is amplified and filtered before being applied to a level detector circuit which operates an alarm. The detecting element may be at the focal point of a concave parabolic or spherical mirror which will provide a single zone having a sensitivity with strongly directional characteristics. Alternatively, the detecting element may be located at the focal point of an array of mirrors which can conveniently be arranged to produce a number of widely spaced radial zones of sensitivity. If these zones are arranged suitably, a large space can be covered by one detecting element.
A disadvantage of both these systems is that their zones of sensitivity can be partially or totally obscured by placing thermally opaque material over their apertures. Such material severely attenuates radiation in the range of wavelengths of interest, which range is typically between 4 and 20 microns and so approximately centred on 10 microns wavelength. If this is done, any change in radiation caused by an intruder will not be detected and the alarm will not be activated. This aspect places a serious limitation on the use of passive infra-red detectors for security purposes.